1. Field of the Invention
The present invention relates to a sequence controller and more particularly relates to a program changeable sequence controller wherein a read-only memory, which is capable of erasing a program therein, is employed as a program memory medium and the loading of the program into the read-only memory and the change thereof are quickly and easily performed.
2. Description of the Prior Art
Conventional sequence controllers are provided with a read-write memory as a major controller memory and therefore, it is possible to directly load a predetermined portion of a program or to change the program. However, the read-write memory is generally a core memory in which the instructions are erased at each read-out operation. Therefore, the program is prone to be damaged due to normal environmental hazards. Furthermore, the required control circuit of such systems is complex and the core memory is expensive.
In another type of conventional sequence controller, a read-only memory in which the program can be erased using either external electrical circuitry, or by the application of ultraviolet light is utilized as a major controller memory. Such sequence controllers, however, are operated by means of a read-write memory for a first operating cycle and, when the program is perfected, the program is transferred to the read-only memory for permanent use. In this case, the capacity of the read-write memory should be made as large as the whole read-only controller memory, which becomes expensive. Additionally, the read-write memory is susceptible to electrical noise and thus the program therein is also prone to be damaged while the controller is operated by means of the read-write memory. Once the program is damaged, the entire program should be corrected or at least examined to test it for failures.
It will be understood by those skilled in the art that it is possible in a program erasable read-only memory to erase all of the program at one time, but impossible to erase a part thereof.